Grease pencil strokes disappear in surface mode in Blender 2.8
I just recently experienced the same problem!

Maybe a solution to your problem:
In the "stroke placement" dropdown if you select "surface" there is also an option for an offset.
This works good.
However I found out that the offset to the projected surface as well as the thickness of the Gpencil is relative to the viewing distance to the object.

There should be an option for relative to object and relative to screenspace if you have placement on surface selected. 
Where can we report this? I don't know how I can get in touch with devs... 

Anyways, I hope that helped!